Tunable magnetrons



July 31, 1962 HORRIGAN 3,047,770

TUNABLE MAGNETRONS Filed Sept. 8, 1960 INVENTOR JOHN HORR/GAN jaw A'ITORNEY tts This invention relates to magnetrons and more particularly to the elimination of suprious modes and deterioration of the power output of a tunable magnetron introduced by the movement of the tuner.

In certain types of tunable magnetrons, the operating frequency of the magnetron is varied by introducing tuning pins into each of the resonator bores. The pins are usually supported on a disc to be moved in unison into and out of the resonator bores, the pins normally extending through openings in the magnet pole piece at the end of the anode block opposite the high voltage bushing. Inasmuch as tuners of this type are designed to encompass a range of travel from a position where the pins barely enter the resonators to a position where they extend completely into the resonator bores, in some positions of the tuner ring there is a relatively large cavity formed between the tuner ring and the upper end of the pole piece. As the tuner is moved to adjust the operating frequency of the magnetron, the cavity formed by the tuner ring and the pole piece goes through resonance conditions which may establish conditions favorable to transmission of radio frequency power in a coaxial mode through the coaxial waveguide formed by the cathode and the central bore of the pole piece. This removal of power from the main cathode anode space may change the spectrum characteristics of the anode, and also reduees the power coupled from the anode.

It is an object of this invention to eliminate spurious modes and coupling of power to the cavity formed by the tuning structure and the pole piece of a tunable margnetron.

It is another object of this invention to shield the variable cavity between the tuning structure and the pole piece in a manner which permits rapid and thorough evacuation of the magnetron tube.

It is a further general object of this invention to provide an improved magnetron.

In the attainment of the foregoing objects, the central axial opening in the pole piece used to center the cathode is closed at the end confronting the tuner ring by a metallic mesh or screen for suppressing the transmission of radio frequency energy along the cathode and into the variable cavity above the pole piece. This screen shorts out the radio frequency power at this point leaving the region above the pole piece free of radio frequency power and eliminating all coupling of the variable cavity to the anode. In addition to the shielding function, a screen across the central opening in the pole piece permits the flow of air therethrough whereby the tube, especially the volume above the pole piece, may be rapidly and freely evacuated.

Other objects, features and advantages of the invention will be better understood from the following detailed description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a sectional view of a magnetron in which this invention may be incorporated; and

FIG. 2 is a plan view of the upper pole piece of the magnetron of FIG. 1 showing the disposition of the screen.

Turning now to the drawing, the magnetron therein partially illustrated comprises an anode having an axial bore 12 and a plurality of cavity resonator bores 14 and 16 equally spaced in a circular array around the tent t 3,@47,77fl Patented July 31, 1962 axial bore 12 and communicating therewith. A cathode 18 comprising a cylindrical sleeve having an electron emissive coating thereon is positioned within the axial bore 12 and supported by a high voltage bushing, partially shown at 20. The upper end of the cylindrical cathode sleeve is centered within the axial bore 22 of the upper pole piece 26, a similar pole piece being contained within the cylindrical structure 20 at the other end of the cathode. Tuning pins 28 and 30 extend into the cavity resonator bores 14 and 16 and are axially insertable therein by a tuning mechanism 32. which includes a circular conductive ring 34 to the periphery of which the tuning pins are secured. The tuning pins 28 and 3t) (and additional pins for the remaining anode resonators) extend through openings 36 and 38 arranged in a circle in the upper pole piece 26 and through openings aligned therewith in end filler 4% Energy from the magnetron is transmitted from one of the cavity resonator bores, bore 16 as illustrated, through a waveguide system comprising an H-section transformer 42 and an additional transformer and matching device well known to the art, and forming no part of the present invention.

Tuning of the illustrated magnetron is accomplished by varying the extent of insertion of the tuning pins into the cavity resonator bores. At the low frequency end of the tuning range the pins are essentially withdrawn from the resonators, and at the high frequency end they extend almost to the bottom of the resonators as viewed in FIG. 1. Thus, in tuning through the complete range, the volume defined by the upper end of pole piece 26 and the tuner ring 34. varies from a volume almost twice that shown in the drawing to essentially zero volume; i.e., with the lower surface of the ring 34 essentially in contact with the upper face of the pole piece. Unless precautions are taken this variable volume cavity becomes coupled to the anode resulting in loss of power into the upper cavity and/ or the generation of unwanted modes in the magnetron. Energy may be transmitted into the cavity along the coaxial waveguide formed by the upper end of the cathode sleeve and the bore of the pole piece, the upper cavity behaving as a radio frequency load on the anode and lowering the Q of the anode. Moreover, with movement of the tuner, the upper cavity goes through resonance conditions Which interfere with the desired mode in the anode, and may actually encourage unwanted modes in the magnetron.

In accordance with this invention, power loss to the tuning head cavity is eliminated by a :fine mesh screen 44 secured over the upper end of the axial bore 24 of the pole piece 26. The metallic screen, preferably of circular shape, is brazed to the pole piece inwardly of the openings 36 and 38 and may be applied as a subassembly step in the manufacture of the tube since its presence on the pole piece does not interfere in any way with the assembly of the pole piece with any of the other parts of the tube structure. The screen is very simple to apply, and obviously much simpler to fabricate than a metal plug or radio frequency choke. This screen suppresses the transmission of radio frequency energy into the cavity above the pole piece with the consequence that adjustment of the tuning structure has no effect on the anode other than, of course, the desired effect of the insertion of the tuning pins.

Besides being a very effective suppressor of transmission of energy into the upper cavity, and simpler and less expensive to fabricate than prior solutions to this problem, a screen offers the further advantage of rapid pumping of the tube. In the illustrated tube, the parts are completely assembled and sealed and then evacuated through a tubulation 46 communicating with the interior of the anode block 10. While the air in the tuning mechanism 32 and the volume between the pole piece 26 and the tuning ring as could be drawn through the openings 36 and 38, an unnecessarily long time for pumping would be required because of the small clearance between the tuning pins and the openings 36 and 38. The screen 44, in effect, leaves the axial bore 24 open for the flow of air from the tuning head cavities, permitting rapid evacuation of the tube.

From the foregoing it is seen that applicant has pro vided a simple and inexpensive means for eliminating the troublesome intercoupling between the tuning head cavities and the anode of a tunable magnetron, which affords the additional significant advantage of rapid evacuation of the tuning head cavities.

What is claimed is:

1. In a tunable magnetron including an annular resonant anode structure, a cathode concentrically positioned within said anode, a pole piece at one end of said anode having a central opening therein coaxial with said cathode, and a tuning structure including a conductive ring having pins extending through said pole piece into said anode structure, said ring being positioned at the end of said pole piece remote from said anode and axially movable relative to said pole piece, a conductive screen secured to the end of said pole piece remote from said anode and covering the central opening therein for preventing transmission of radio frequency power between said anode structure and the region between said tuning ring and said pole piece, said screen being of sufiiciently open mesh to permit the evacuation of air from the region between said tuning ring and said pole piece through said central opening in said pole piece.

2. In a tunable magnetron including a resonant annular anode structure, a cylindrical cathode supported concentrically within said anode and extending beyond one end of said anode, a cylindrical magnetic pole piece supported at said one end of said anode having a central opening therein coaxial with and surrounding the extension of said cathode, and a tuning structure including a circular conductive ring having pins secured to the periphery thereof and extending through axial openings near the periphery of said pole piece into said anode structure, said ring being axially movable relative to said pole piece to effect tuning of said anode structure whereby a cavity of variable dimensions is formed between said ring and the end of said pole piece remote rom said anode, a conductive screen of sufficiently open mesh to permit the free passage of air therethrough conductively secured to the end of said pole piece remote from said anode and covering said central opening therein to prevent coupling of radio frequency power between said anode structure and said cavity.

References Cited in the file of this patent UNITED STATES PATENTS 2,906,920 Crapuchettes Sept. 29, 1959 

